Heretofore, electrical filters have been widely used to control the flow of electrical signals that travel through a circuit. In receiver front-ends, preselector filters have been used to pass frequencies associated with a particular channel and to reject other frequencies before they pass to the receiver. Such preselector filters have been designed to control bandwidths ranging from a fraction of a hertz in control systems, to 3 kilohertz in telephone systems, to 20 kilohertz in AM radio systems, to 6 megahertz in television systems.
In microwave and millimeter-wave applications, planar microwave filter structures such as microstrip bandpass filters and stripline filters have been used to achieve such electrical filter functions. Microstrip filters are formed by etching a circuit pattern on one side of two metal layers separated by a dielectric substrate. Stripline filters, on the other hand, are formed by etching a circuit pattern on a metal layer that is sandwiched between two dielectric slabs, each having an outer surface covered by a metal ground plane. To achieve electrical filtering in such microstrip and stripline systems, the circuit pattern etched on their respective metal layers is essentially a series or array of resonant metallic elements. These metallic elements have a length that is one-half the guide wavelength at the center frequency of the passband of the preselector filter. Moreover, the resonant elements are spaced in relation to one another such that only the frequencies within the passband pass through the stripline or microstrip circuit. As a result, the circuit pattern etched in the microstrip or stripline acts as a wideband fixed-tuned bandpass filter.
In some RF applications, however, there is often a need to reject or excise a selected frequency or a narrowband of frequencies within the passband which are considered interference. In an effort to reduce or eliminate these undesirable frequencies, tunable notch filters were developed. One such notch filter, using an yttrium-iron-garnet (hereinafter YIG) resonator (i.e. sphere), was developed to enable a receiver to reject any frequency within the passband of the microwave or millimeter-wave system front-end.
These YIG filters, however, are not planar devices and thus are not compatible with microwave and millimeter-wave integrated circuitry. Moreover, RF connectors must be mounted to the YIG filter housing. As a result, discrete magnetically-tunable YIG filters can be costly and bulky when utilized in microwave or millimeter wave systems as preselector circuits.